Tecplot's macro language does have an IF statement, but it only works on variables that are declared within the macro, using the VARSET statement.

You can access the data you have loaded into Tecplot from the macro language, but you have to do it using function calls. Specifically, you can fetch a value of a variable for a given zone using the GETFIELDVALUE function, and you can set a value of a variable in a given zone using the SETFIELDVALUE function.

You would need to check all of the values of the desired variable in a given zone, and there is no way to check them all at once, so you will need to cycle through all the values for the desired variable in the desired zone one at a time in a loop (the macro language has LOOP and ENDLOOP statements). You can use the GETFIELDVALUE and SETFIELDVALUE statements within the loop to fetch each value and then reset it if you need to.